After a user has lifted a heavy load with the ejection spindle of a conventional hydraulic jack, the ejection spindle takes much time descending to its initial position.
Furthermore, an oily liquid for use in the hydraulic pipeline inside the jack is susceptible to contamination, and thus the hydraulic pipeline inside the jack is prone to congestion which blocks the passage of the oily liquid, thereby rendering the jack inoperable.
Furthermore, if the hydraulic pipeline is clogged and the user keeps feeding the oily liquid into the jack, not only will the jack become inoperable, but the jack will also get damaged because of the difference in the cross-sectional area between a slave cylinder and a master cylinder of the jack.
Referring to FIG. 1, unlike the aforesaid conventional hydraulic jack, a conventional double-acting jack is characterized by an oil feeding hole 1 through which oil is fed to a primary oil chamber 2 in order to elevate an ejection spindle 3 of the double-acting jack. Referring to FIG. 2, oil is fed via a descent oil route 4 to a secondary oil chamber 5 disposed in the ejection spindle 3, so as to lower the ejection spindle 3. The conventional double-acting jack is advantageously characterized in that the ejection spindle 3 of the double-acting jack is capable of quick restoration to its initial position. However, the conventional double-acting jack has the following drawbacks: the primary oil chamber 2 has a much larger cross-sectional area than the secondary oil chamber 5 such that, due to the large difference in the resultant pressure between the primary and secondary oil chambers 2, 5, over-compression is likely to occur to thereby damage the double-acting jack whenever the hydraulic pipeline in the double-acting jack is clogged and the user keeps feeding oil to the primary oil chamber 2 or the secondary oil chamber 5.
Accordingly, it is imperative to provide a jack conducive to reduction in the likelihood of pipeline congestion and over-compression and conducive to quick restoration to its initial condition.